A 3D printing technology is a technology that is opposite to the conventional material processing method and constructs a 3D object by overlapping materials layer by layer based on 3D Computer Aided Design (CAD) model data. The 3D printing is based on a 3D model, takes materials like a high molecular plastic, a photosensitive resin, a metal powder and a film coating powder as raw materials and implements the manufacturing of the 3D model in a principle of overlapping the materials layer by layer. The 3D printing technology is a combination of a CAD, a mechanical automation and other technologies, such that the manufacturing process is simplified, a development period of a new product is shortened and the development cost and risk are reduced. It is considered as a third industrial revolution.
In the technical field of the 3D printing, an SLM forming technology is one of the technologies with the most development potential. It directly melts a metal powder using a high-energy beam laser and forms a high-performance metal part. During the SLM forming process, a 3D model of a part is designed on a computer first, then the 3D model is sliced and layered to obtain Two-Dimensional (2D) contour data of each cross section, and thereafter a processed data is imported to a forming device. When the forming device works, a feed container is first lifted to a certain height, a layer of the powder is spread on a build cylinder uniformly by a recoater, and the computer controls a 2D scanning trace of a laser beam according to a prototype slicing model such that a metal powder material is selectively melted and solidified to form a layer of the part. After the contour of this layer is scanned completely, the above steps are repeated to perform laser melting forming of a next layer till the whole part is processed completely to form a 3D metal part. The technology can manufacture a metal product with a complex shape, makes a formed part have good performance and high precision and therefore has important applications in fields such as medicine, aerospace and military.
At present, during the SLM forming process, there still exists the following problems to be solved.
{circle around (1)} A substrate is difficult to level automatically. To reduce the warping deformation of the part, the product is generally sintered on the substrate such that the product and the substrate are formed into one whole structure. Currently, the substrate is mainly leveled manually by a feeler gauge, so an adjustment process is troublesome, the accuracy is low and the forming efficiency is reduced.
{circle around (2)} The porosity of the spread powder and the product is difficultly monitored. During the forming process, the internal porosity of the spread powder and the product directly affects a property of the product. Currently, during the SLM forming process, the internal porosity of the spread powder and the product cannot be detected. If there are more unqualified pores inside the product, the forming will be performed again, such that not only the forming efficiency is reduced, but the material is also wasted.
{circle around (3)} During the forming process, the melting pool is difficultly monitored. During the forming process, the stability of the melting pool directly affects the dimensional accuracy and the mechanical performance of the formed product. During the SLM forming process, the melting pool is largely affected by factors such as a laser power, a beam scanning speed, a gas component, an air flow, a material performance and a layer thickness. Hence, there is a need for a monitoring system to have a fast shooting speed and a relatively large data volume and to implement signal processing. The common monitoring devices cannot implement real-time monitoring.
{circle around (4)} The formed part is easy to occur warping deformation. During the forming process, a thin-walled portion of the formed part is cooled rapidly, such that an internal temperature is distributed non-uniformly, the warping deformation of the formed part is caused easily, the product performance is reduced or even a waste part is formed; and thus, the rate of finished products is reduced.